As is known, a practical problem related to infrared detection in a silicon (Si) photodiode is the difficulty of generating and collecting charges, as the low energy of the infrared radiation reduces the chance of electron transition from Si valence band to Si conduction band. For example, 30 μm silicon depth is required to convert 90% of the light at 830 nm wavelength. Furthermore, since so deep photodiodes are not easy to be realized, electrons photo-generated in the deepest portion of a Si photodiode diffuse and are collected by adjacent photodiodes, thereby causing cross-talk.
Therefore, current photodiodes sensitive to NIR radiation are manufactured with a thick Si epitaxial layer having low doping, in order to increase the chance of collection of photo-generated charges. Once generated, said charges diffuse in all directions and only some of them are collected by the photodiodes located where the photons impinge.
It is also possible to introduce a doping gradient in the Si bulk substrate so that electrons drift towards the Si surface; however, the doping concentration must be low enough in order to avoid recombination of electrons along the path; furthermore, this cannot avoid that electrons generated by a photon impinging on a photodiode may be collected by an adjacent photodiode (cross-talk).
Moreover, also germanium (Ge) and silicon-germanium (SiGe) alloys have been used, instead of Si, as photosensitive films/layers in NIR photodetectors due to their higher absorption coefficients in NIR wavelength spectrum. Conventionally, Ge layers are deposited by means of chemical vapor deposition (CVD), physical vapor deposition (PVD) or molecular beam epitaxy (MBE), but a Ge high-quality epitaxial film is difficult to be obtained.
In this respect, U.S. Pat. No. 7,008,813 B1 discloses a process for growing a good quality Ge epitaxial layer on a Si wafer by using a liquid phase epitaxy (LPE) process. In detail, U.S. Pat. No. 7,008,813 B1 describes a method of fabricating a germanium photodetector, that includes: preparing a silicon substrate having a layer of silicon nitride thereon; depositing a first germanium layer on the silicon nitride layer so that a portion of said germanium layer is in direct physical contact with the silicon substrate; encapsulating the germanium layer with a layer of silicon oxide; annealing the structure at a temperature such that the germanium melts and the other layers remain solid, wherein a rapid thermal annealing (RTA) is performed at 900-1000° C.; and growing a second, single-crystal layer of germanium on the structure by liquid phase epitaxy (LPE). According to U.S. Pat. No. 7,008,813 B1, when a single crystalline germanium is formed, the defects are concentrated at the silicon-germanium interface where the growth front begins.
Another US patent, namely U.S. Pat. No. 7,157,300 B2, discloses a method for fabricating a germanium-based IR sensor, which method solves the problem of high temperature need for germanium integrated circuits (IC) processes, i.e., ion implantation activation processes are usually performed following an annealing at about 800° C. Such high temperature processes degrade the quality of germanium thin films, because the thermal expansion coefficient of silicon, germanium and silicon dioxide are different. A high temperature process performed on a Ge thin film bonded to a Si wafer usually results in defects in the Ge layer(s), as described in U.S. Pat. No. 6,645,831 B1. In particular, the method according to U.S. Pat. No. 7,157,300 B2 exploits a Ge thin film directly bonded to a Si bulk substrate to preserve the quality of the Ge crystal layer by avoiding the need for high temperature processes after germanium bonding.
Attempts have been also made to directly deposit SiGe alloys on silicon to fabricate an IR detector. However the SiGe has a different lattice cell size compared to the silicon one. This fact causes a not negligible stress when a SiGe layer is grown/deposited on top of silicon. This lattice mismatch limits the maximum SiGe thickness which may be formed. Germanium has a high IR absorption coefficient, however a 4% lattice mismatch to silicon results in a high dark current when a germanium photodetector is fabricated by direct deposition of germanium on silicon.
In this connection, U.S. Pat. No. 7,786,469 B2 describes a thermal sensor with a SiGe superlattice structure formed on an SOI (Si-on-insulator) wafer and bonded to a Si wafer where a CMOS readout circuitry is fabricated. In particular, after a proper surface cleaning, the SOI wafer is loaded into SiGe deposition system which can be MBE, CVD or plasma CVD.
Additionally, KR 2006 0122257 A discloses a method for manufacturing a photodiode and an image sensor to improve light receiving capability and to fabricate the photodiode on a cheap silicon substrate. In particular, according to KR 2006 0122257 A, germanium ions are implanted into a photodiode forming region. The implanted germanium ions are annealed. A Shallow Trench Isolation (STI) is formed in a silicon substrate. N-type dopants are implanted into the photodiode forming region. P-type dopants are then implanted into the photodiode forming region, thereby forming a photodiode on the cheap silicon substrate.